Boiler systems are widely used industrially and in residential application for raising steam or supplying hot water. Steam raising systems may supply saturated steam or superheated steam. In all boiler systems and their associated equipment, including piping, valves, heat exchangers, radiators, etc., the heated water or steam is circulated in a loop which may be open or closed. Most industrial boiler and residential boiler systems are constructed of carbon steel and equipment corrosion is a continuing and serious problem which has to be addressed both with proper equipment design and operation along with periodic inspection and maintenance.
The dissolved gases normally present in water cause many corrosion problems. For instance, oxygen in water produces localized pitting while carbon dioxide corrosion is frequently encountered in condensate systems and less commonly in water distribution systems. The resulting corrosion may lead either to removal of material as with pitting or to deposit formation on heat transfer surfaces to reduce efficiency and reliability.
A number of measures may be taken to inhibit corrosion or its effects on the system. Proper feed-water selection and treatment, for example, by deaeration, can contribute materially and chemical treatment can inhibit the direct effects of corrosion-inducing species or of the corrosion products which may be formed. Typical treatment chemicals include neutralizing amines, filming amines, and oxygen scavenger-metal passivators. Neutralizing amines are used to neutralize the acids generated by the dissolution of carbon dioxide or other acidic process contaminants in the water. Filming amines, by contrast, act by forming a protective film on metal surfaces which replaces loose oxide or sulfide scale present on the surfaces with a thin amine film barrier. Filming amines have, however, a tendency to form deposits by reacting with multivalent ions, such as sulfates, and certain metals frequently present in water supplies and so require addition in controlled amounts.
Filming amines containing one ingredient, such as hexadecylamine, octadecylamine and dioctyldecylamine are effective but often fail to cover the entire system and can produce fouling. Emulsifiers and, in some cases, small amounts of neutralizing amines can be added to improve film distribution by providing more uniform coverage and reducing fouling potential. Treatment with a combination, e.g., filming and neutralizing amines with dispersant aids, may provide superior corrosion protection.
One factor in the use of amine corrosion inhibitors is the distribution between the steam and water phases. In closed system in which both steam and water will be present, the distribution of the amine between the steam and liquid phases is significant: as the steam condenses, acidic contaminants can either remain in the steam or dissolve in the liquid phase. Some contaminants, such as carbon dioxide, stay mainly in the steam phase while others go into the liquid phase. Amines that are more likely to distribute into the steam include cyclohexylamine but if it is used in a system with two condensation zones in series, acidic corrosion agents may deposit/condense into the liquid phase at the first condensation site while the cyclohexylamine will tend to remain in the steam. This results in low pH in the first condensation site liquid phase. At the second site, where total condensation takes place, the pH is high.
Thus, there is a considerable number of factors entering into the choice of amine corrosion inhibitors in boiler systems and additional amines and amine systems are desirable in this application.